1. Field of the Invention
The present invention relates to a charging method of performing charging of a secondary battery and to a charging apparatus for carrying out such a charging method.
2. Description of the Related Art
In general, a secondary battery has the following charge characteristic. As a charging current is supplied to a secondary battery, the voltage thereof gradually rises as shown in FIG. 1(a) and then drops after it has reached a peak as shown at a point P in FIG. 1(a). For this reason, in a conventional charging apparatus, the voltage of the secondary battery is monitored during charging, and if it is detected (at a point A in FIG. 1(a)) that the voltage dropped by a predetermined value .DELTA.V (negative delta voltage: -.DELTA.V) after it had reached the peak, it is determined that the secondary battery has been fully charged, and the charging operation is completed.
However, all secondary batteries do not exhibit the charge characteristic shown in FIG. 1(a). For example, if secondary batteries remain not in use for a long period, some of them will exhibit a charge characteristic in which a voltage peak (P' of FIG. 1(b)) appears within a short time after the supply of a charging current has been initiated, and the voltage further rises after it has dropped once. If a secondary battery having such a charge characteristic is to be charged by means of the conventional charging apparatus, the voltage of the secondary battery drops by .DELTA.V at a point B in FIG. 1(b), at which time it is determined that full charge has been achieved, and the charging operation is interrupted. As a result, the charging operation is brought to an end in spite of an insufficiently charged state.